build: establish profile-aware OpenGL headers

[tritao]

Summary

Establish the profile-aware OpenGL header and build-configuration foundation used by the rest of the renderer stack.

• Vendor pristine Khronos core OpenGL and WGL headers privately, with exact upstream provenance.
• Keep Inventor/system/gl.h as Coin's public OpenGL abstraction and isolate compatibility enum fallbacks in gl-fallbacks.h.
• Select private core-profile headers only while compiling Coin; installed consumers continue to use normal platform OpenGL headers.
• Add explicit LegacyGL build configuration while keeping compatibility rendering enabled by default.
• Publish the installed COIN_HAVE_LEGACY_GL_RENDERER capability separately from the build option.
• Validate the installed CMake package through a standalone external consumer.
• Add initial LegacyGL-disabled core-header/glue coverage without claiming that the complete library is optional yet.
• Allow CI and CodeQL to run on stacked pull requests.

This layer establishes header and configuration policy only. It does not yet make LegacyGL traversal or implementation units optional.

Stack

Layer 1/18, based on freecad-master.

[Lgt2x]

Was the target branch change intentional @tritao ? The diff is borked

[tritao]

Was the target branch change intentional @tritao ? The diff is borked

Not intentional, it got screwed up somehow, fixed now.

[Lgt2x]

it changed again @tritao :(

[tritao]

Fixed again.

[tritao]

Thanks for the detailed review. I addressed the feedback as follows:

• Kept the Khronos headers private and byte-for-byte identical to the recorded upstream revision.
• Kept installed consumers on normal platform OpenGL headers.
• Simplified the profile-aware header selection.
• Left basic.h.in unchanged (we will remove it with that other Automake cleanup PR later).
• Moved COIN_BUILDING_COIN to gl: add profile-neutral GLSL dispatch and core test support #65 (never intendeded for that change to end up here in the first place)
• Switched CI to Ninja and parallel builds.
• Kept pull_request: {} as the explicit unfiltered trigger.
• Documented the standalone installed-package smoke test.
• Simplified comments and corrected the PR description and base.